Mineral or synthetic oil lubricants, such as driveline fluids, engine oils, or automatic transmission fluid (ATF), do not necessarily function the same over wide temperature variations. Such lubricants, for instance, may become less effective at high temperatures because heat reduces their viscosity and film-forming ability. Alternatively, lubricants can also become less effective at cold temperatures as the viscosity of the lubricant increases. This problem is common to many oil lubricants and can be characterized in terms of “viscosity index” (VI), which is an arbitrary measure for the change of viscosity of a lubricating oil with variations in temperature. The lower the VI, the greater the change in viscosity of the oil with temperature changes and vice versa. The viscosity of a lubricant is closely related to its ability to reduce friction. Generally, the least viscous lubricant which still forces two moving surfaces apart is desired. If the lubricant is too viscous, it will require a large amount of energy to move the surfaces; if it is too thin, the surfaces will come in contact and friction will increase. Many lubricant applications, such as lubrication for engine oils, driveline fluids, or automatic transmission fluids, require the lubricant to perform consistently across a wide range of temperatures. However, many lubricants do not inherently have a high enough VI to remain consistent across the wide range of temperatures required by an automobile.
In an attempt to address this shortcoming, a viscosity index improver (“VII”) can be added to the lubricant. Viscosity index improvers are commonly polymers, and are added to reduce lubricant viscosity changes at high and low temperatures. When viscosity index improvers are added to low-viscosity oils, they effectively thicken the oil as temperature increases. This means the lubricating effect of mineral oils can be extended across a wider temperature range.
In some instances, automatic transmission fluids (ATF) help with the minimization of shudder (that is, anti-shudder properties), which is believed to be a function of the change of friction coefficient with time (dμ/dt<0) of the transmission. Furthermore, shift characteristics of automatic transmissions are primarily dependent on the frictional characteristics of the ATF. The ATF fluid typically needs to have a high and stable frictional performance over the life of the fluid, good anti-shudder performance, and anti-wear characteristics over a broad temperature range. These characteristics are often a challenge to balance with the requirement that today's ATF lubricant compositions also need to maximize service intervals, or even better, avoid oil service during the lifetime of the equipment. This is referred to in the industry as a lifetime fill or “fill-for-life” fluid. Therefore, maintenance of the friction properties of an ATF over time, i.e. friction durability, may also be a desired property of the fluid.
Several prior efforts at improving the friction properties of lubricating oils have been attempted, including the addition of, or increased levels of, lubricant components such as viscosity index improvers. The addition of or increased levels of components can escalate manufacturing complexity and increase product costs. While poly(meth)acrylate (PMA) additives have been used as VIIs, known examples have advanced structures, rely on advanced technology that drives up the cost of manufacturing, and only have a moderate effect on viscosity index.